Lighting solutions using light emitting diodes (LEDs) are gaining in popularity because of the low power consumption required to produce a given level of light output, their efficacy, compared to other lighting technologies. LEDs, being diode, require a certain amount of forward voltage before they begin to conduct current. Once the junction is biased to allow current to flow through the LED, changing the current through the LED changes the light output of the LED. Accordingly, it is common to drive an LED lamp using a constant current driver that outputs a constant, or maximum direct current (DC) level.
One of the issues that has arisen with LED lamps is that as an LED changes temperature, then the semiconductor properties also change, which can cause changes in light output, such as total lumen and chromaticity. In general, as the junction temperature increases, light output level decreases at a given drive current. This results in the light output of a LED lamp being brighter when it is initially turned on, and decreasing during operation as the junction temperatures of the LEDs increase. The change in light output may be perceived by people, particularly if different LED lamps are turned on at different times near each other.
Another issue is that LED lamps are typically powered by voltage regulated power supplies. However, the forward operating voltage of LEDs can vary depending on the manufacturing process. As a result, the forward operating voltage of LEDs in a given LED lamp can vary substantially. Therefore, either the voltage regulated power supply must supply enough voltage to ensure that the LED lamp will have enough voltage for a worst case condition where all the LEDs have a high forward operating voltage, or the LEDs must be sorted to ensure they all have the same or similar forward operating voltage. Having excess overhead voltage is inefficient, and testing and sorting LEDs by forward operating voltage adds expense to the cost of manufacturing the LED lamp.
Accordingly, there is a need for a method and apparatus for maintaining the light output of a LED lamp constant over temperature.
Those skilled in the field of the present disclosure will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. The details of well-known elements, structure, or processes that would be necessary to practice the embodiments, and that would be well known to those of skill in the art, are not necessarily shown and should be assumed to be present unless otherwise indicated.